Engine table name prefix generator fix

[morgoth]

When generating namespaced model inside mountable engine, table_name_prefix was not including engine name:

cd my_engine
rails g model namespaced/user

File: my_engine/app/models/namespaced.rb:

module MyEngine
  module Namespaced
    def self.table_name_prefix
      'namespaced_'
    end
  end
end

Table name prefix should include engine name: my_engine_namespaced_

I added failing test and fix.
I noticed that one test failed after this change, which I believe was wrong assumption.
I also fixed this test according to current changes

[rafaelfranca]

cc/ @drogus